The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with gradient coils for a magnetic resonance imaging apparatus and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with localized magnetic resonance spectroscopy systems and other applications which utilize gradient magnetic fields.
In magnetic resonance imaging, a spatially uniform and temporally constant magnetic field is created through an examination region in which a subject to be examined is disposed. A series of radio frequency pulses and magnetic field gradients are applied to the examination region. Gradient fields are conventionally applied as a series of gradient pulses with pre-selected profiles. The radio frequency pulses excite magnetic resonance and the gradient field pulses phase and frequency encode the induced resonance. In this manner, phase and frequency encoded magnetic resonance signals are generated.
Many MRI techniques are highly sensitive to magnetic field homogeneity. However, the geometric shape and/or magnetic susceptibility of a subject being scanned, built-in main magnet tolerances, environmental and/or site effects, and the like contribute to the main magnetic field""s inhomogeneity and/or non-uniformity. In turn, this leads to imaging problems.
Methods for controlling the homogeneity of the main magnetic field include both passive and active shimming techniques. The passive technique includes arranging shim steel on the inside diameter of the superconductive coil assembly to minimize static magnetic field inhomogeneities based upon NMR field plot measurements. The active shimming technique generally employs multiple orthogonal shim coils and/or gradient coil offsets. An electrical current is applied to the shim coils and/or gradient coil offsets in order to cancel inhomogeneities in the main magnetic fields.
Gradient magnetic field pulses are typically applied to select and encode the magnetic resonance with spatial position. In some embodiments, the magnetic field gradients are applied to select a slice or slab to be imaged. Ideally, the phase or frequency encoding uniquely identifies spatial location. In bore-type magnets, linear magnetic field gradients are commonly produced by cylindrical gradient field coils wound on and around a cylindrical former. Discrete coils are wound in a bunched or distributed fashion on a similar or larger diameter cylindrical tube, commonly 30-65 centimeters in diameter or larger.
Historically, gradient coil designs were developed in a xe2x80x9cforward approach,xe2x80x9d whereby a set of initial coil positions were defined and the fields, energy, and inductance calculated. If these quantities were not within the particular design criteria, the coil positions were shifted (statistically or otherwise) and the results re-evaluated. This iterative procedure continued until a suitable design was obtained.
Recently, gradient coils are designed using the xe2x80x9cinverse approach,xe2x80x9d whereby gradient fields are forced to match predetermined values at specified spatial locations inside the imaging volume. Then, a continuous current density is generated which is capable of producing such fields. This approach is adequate for designing non-shielded or actively shielded gradient coil sets.
Conventional shielded gradient coil sets have been designed based on the assumption that no correction to the magnitude of the higher order gradient magnetic field terms was to be made. Therefore, conventional gradient coil designs force field contributions from terms of higher spatial order to be negligible within the imaging volume. This is usually accomplished by setting specific moment coefficients to be zero. Although such designs enhance the uniformity of the gradient magnetic field with a subsequent improvement in the distortion characteristics of the final image, they are characterized by high inductance and resistance values, which result in a significant reduction in the gradient coil""s peak magnetic field, rise time, slew rate, increase in heating characteristics, and reduction in duty cycles.
Conversely, design requirements call for fast switching, high duty cycles, and high peak gradient coils force the introduction of higher order, non-zero terms in the gradient magnetic field. These higher order terms distort image quality by introducing spatial misregistration of spins. Passive distortion correction algorithms have been implemented for the correction of such image distortions. In particular, two distortion correction methods have been employed.
One method includes the use of specially designed RF pulses with a field intensity variance to account for the distortion characteristics of the gradient magnetic field. The pulses are applied prior to pulsing the gradient magnetic field during the MR sequence. One disadvantage of this technique is that it is limited to a two-dimensional MR sequence with limited oblique capabilities. In addition, a signal intensity correction algorithm is necessary during the postprocessing of the MR image.
A second prior art method of gradient distortion correction employs a postprocessing correction algorithm that predicts the image distortions (pixel or voxel displacements) generated by the higher order terms of the gradient magnetic field and attempts to invert this action. However this technique suffers from several disadvantages. First, this technique is ineffective in eliminating aliasing when the gradient coil""s rollover point is inside the field of view. Second, a signal intensity correction algorithm must be applied to the undistorted image. Also, the technique is ineffective for gradient field non-uniformities exceeding 35% of the gradient field""s ideal value over any imaging volume.
Therefore, a need exists for a gradient coil set with correctable higher order terms. The present invention contemplates a new and improved gradient coil set which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a magnetic resonance apparatus includes a main magnet which generates a main magnetic field through and surrounding an examination region, where the main magnetic field contains inhomogeneities. A gradient coil assembly generates gradient magnetic fields across the examination region, where the gradient magnetic fields include higher order harmonics. A multi-axis shim set is disposed adjacent the gradient coil assembly. The multi-axis shim set cancels inhomogeneities in the main magnetic field and compensates for distortions caused by the higher order harmonics of the gradient magnetic field. An RF transmitter and coil assembly excites magnetic resonance dipoles in and adjacent the examination region. An RF coil and receiver assembly receive and demodulate magnetic resonance signals from the resonating dipoles. A reconstruction processor reconstructs the demodulated magnetic resonance signals into an image representation.
In accordance with another aspect of the present invention, a method of magnetic resonance imaging includes generating a temporally constant main magnetic field through an examination region, where the main magnetic field contains inhomogeneities. Resonance is induced in selected dipoles in the examination region such that the dipoles generate magnetic resonance signals. A gradient magnetic field, having higher order harmonics which cause image distortions, is generated across the examination region to encode the magnetic resonance signals along at least one axis. Correction currents are applied to shim coils to (i) minimize the inhomogeneities in the main magnetic field and (ii) compensate for the higher order harmonics in the gradient magnetic field. The encoded magnetic resonance signals are received, demodulated, and reconstructed into an image representation free from distortions caused by the higher order harmonics of the gradient magnetic field.
In accordance with another aspect of the present invention, a temporally constant main magnetic field having inhomogeneities is generated in an examination region. Radio frequency pulses excite and manipulate resonance of selected dipoles in a subject disposed in the examination region. Gradient pulses are applied to at least one gradient coil assembly to generate gradient magnetic fields for encoding the excited resonance, where the gradient magnetic fields have inhomogeneities caused by higher order harmonics. Received and demodulated resonance signals are reconstructed into an image representation. A method of real-time correction of the main and gradient magnetic field inhomogeneities includes applying DC correction currents to a multi-axis shim set and superimposing AC correction current pulses on the DC correction currents.
One advantage of the present invention is that it eliminates aliasing effects for magnetic resonance sequences with large fields of view.
Another advantage of the present invention resides in real-time correction of distortions caused by higher orders of the gradient magnetic field.
Another advantage of the present invention is that it eliminates the need for a distortion correction algorithm.
Another advantage of the present invention is that it provides for gradient coil designs of a prescribed dimension which cover a prescribed imaging volume with reduced inductance and increased efficiency.
Another advantage of the present invention is that it reduces post-processing time for a magnetic resonance image.
Yet another advantage of the present invention resides in the elimination of artifacts from distortion characteristics of the magnetic field gradient.
Another advantage of the present invention is that it reduces overall time for a magnetic resonance study.
Still another advantage of the present invention is that increases the effective imaging volume.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.